On occasion, populations, such as lake trout Salvelinus namaycus in Lake Ontario are extirpated by chronic exposure to chemical contaminants (Cook et al., 2003). Alternatively, populations may develop resistance (increased tolerance) when exposed to chemical contaminants as a result of genetic adaptation or physiological acclimation. However, the acquisition of resistance is not a predictable consequence of exposure to metals, aromatic hydrocarbons, or other toxicants. Ecosystem-specific exposure conditions and life history characteristics of challenged populations probably determine whether acute toxicities occur or resistance develops (Wirgin and Waldman, 2004). Physiological acclimation develops when resistance in an individual is increased after exposure to low levels of a toxicant and will disappear after it is returned to clean environments. In contrast, genetically-based adaptation results from selection pressure on populations in polluted environments and will persist (at least for several generations) after the organisms are placed in clean water. Selectionoccurs onpreexistingpolymorphisms at genes that may ameliorate the effects of toxicant exposure. The existence of genetic resistance in a population is evidence that the toxicant is exerting effects on the population level (Luoma, 1977). Resistance may also have deleterious effects on the community level by increasing the bioavailability of contaminants to higher levels in the food chain. Individuals in a population will vary in their resistance (Courtenay et al., 1994; Nacci et al., 2002a), and the population tolerance distribution in an impacted population can be shifted toward increased resistance, since those individuals have the highest fitness in such an environment. Although there have been many reports of resistant populations frompolluted sites around the world, inmost cases, a distinction between physiological acclimation or genetic adaptation was not made.
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